Forging method, molding device for forgings, and tripod uniform motion universal joint

ABSTRACT

After molding a product-to-be-preliminarily-molded to form a preliminarily molded product as a material, the preliminarily molded product is put into a full-enclosed forging die so that a product is molded with the full-enclosed forging die. On a preliminary leading-end surface to be formed as a leading end surface of a shaft portion of the product formed of the preliminarily molded product, there is formed a shear-drop suppressing portion for suppressing thickness reduction on a boss-portion side of an outer peripheral portion of the leading end surface of the shaft portion of the product. At a time of putting the preliminarily molded product into the full-enclosed forging die so as to mold the product, a relief portion is formed between the leading end surface of the shaft portion to be formed and a cavity of the full-enclosed forging die.

CROSS REFERENCE TO RELATEED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 13/129,650filed May. 17, 2011 which is a the US national phase of PCT applicationPCT/JP2009/070105 filed Nov. 30, 2009, and claiming priority to Japanesepatent application 2008-325955 filed on Dec. 22, 2008.

TECHNICAL FIELD

The present invention relates to a forging method, a forged-productmolding apparatus, and a tripod type constant velocity universal joint.

BACKGROUND ART

In order to form, through full-enclosed forging, a product including aboss portion radially provided with shaft portions, such as a trunnion(tripod member) for constant velocity universal joints and a crossspider for universal joints, a full-enclosed forging die is used.

As illustrated in FIG. 9, the full-enclosed forging die includesopenable/closable dies 1 and 2 and punches 4 and 5 arranged so as to beoperable on a central axis of the dies 1 and 2. Specifically, throughpressing with the punches 4 and 5 under a state in which the dies 1 and2 are closed, a cavity 9 is formed, which corresponds to shapes of shaftportions 7 and a boss portion 8 of a product 6. Thus, as illustrated inFIG. 10A, a billet (material) 10 (refer to FIG. 10A) is put into thecavity 9, and then clamping is performed. After that, the billet 10 ispressed with the punches 4 and 5 so as to be plastically deformed. As aresult, as illustrated in FIG. 10B, the product 6 can be formed, whichis provided with the boss portion 8 and the shaft portions 7.

Specifically, through forging of the billet 10 put into the dies 1 and2, the billet 10 having a columnar shape and a curvature radius R2 asillustrated in FIG. 10A, the product 6 can be formed, which includes theshaft portions 7 each having a leading end surface 7 a with a curvatureradius R2′ larger than R2.

By the way, in the full-enclosed forging die, processing load sharplyincreases in a sealed state, which may lead to a risk of breakage of thedie and shortening of a life thereof. As a countermeasure,conventionally, there has been proposed to set a shaft-molding portionto be longer than a required shaft-portion length so that a reliefportion is provided at a shaft-leading-end portion (Patent Literature1).

However, in the conventional die including the shaft-molding portion inwhich the relief portion is formed, a material obtained throughextrusion of the billet pressed with the punches into the shaft portionis liable to be fluidized on a central portion at the leading endsurface of the shaft portion and is less liable to be fluidized on aperipheral portion thereat. As a result, a shaft portion is formed,which has a leading end surface with a curvature radius smaller than acurvature radius of a leading end surface of a regular shaft portion. Inthis manner, in the conventional die provided with the relief portionformed at a shaft molding portion, there occurs “shear drop” whichrepresents retraction of an outer peripheral side toward a proximal endside in a shaft-portion axial direction relative to the leading endsurface of the regular shaft portion.

Thus, in order to secure a length of the shaft portion while forming theshaft portion with high accuracy with the die, an extra material isrequired by an amount of the “shear drop”. By the way, products moldedby forging with the full-enclosed forging die constitute inner jointmembers for constant velocity universal joints and universal joints.Thus, in order to compactify and weight-reduce the constant velocityuniversal joints and the universal joints using the products, it isnecessary to remove a leading end of the shaft portion by a machiningprocess.

Further, in order to prolong lives during use of the constant velocityuniversal joints and the universal joints in which the products areincorporated, and to suppress vibration and noise, it is required toenhance strength and hardness of the products by heat treatment, and toform shaft-portion outer peripheral surfaces of the products withaccuracy higher than that in a forging process. Thus, it is necessary tofinish the products with high accuracy by a machining process after theheat treatment. In anticipation of the machining process after the heattreatment, in some cases, the shaft leading end is removed by amachining process prior to the heat treatment so that an intersectionsurface of a removal surface and the shaft-portion outer peripheralsurface is used as a reference plane for phase determination at the timeof a high-accuracy machining process on the shaft-portion outerperipheral surface. Therefore, it is necessary to form the intersectionsurface with high accuracy.

Under the circumstance, conventionally, there has been proposed toperform, prior to a step of molding a product shape (referred to asmid-molding), a preliminary molding step of increasing a curvatureradius of a part corresponding to the shaft portion at the time ofmid-molding. With this, at the time of molding of the shaft portion inmid-molding, shear drop can be reduced even under a state in which thecentral portion of the shaft portion is liable to be fluidized incomparison with the peripheral portion thereof and in which the materialdoes not comes into contact with the relief portion. As a result, theconstant velocity universal joints and the universal joints can becompactified and weight-reduced (Patent Literature 2).

CITATION LIST Patent Literature

[PTL 1] JP 2003-343592 A

[PTL 2] JP 2008-188617 A

SUMMARY OF INVENTION Technical Problems

As described above, in the full-enclosed forging die designed to performthe preliminary molding step, the above-mentioned “shear drop” can bereduced. However, when weights of raw materials to be put-into vary fromeach other, an excessively-heavy raw material is mixed in some cases. Insuch a case, the leading end surface (shaft leading end) of the shaftportion comes into contact with the dies so as to cause a sealed state.As a result, processing load sharply increases, which may lead to such arisk that a life of the die is shortened.

In order to solve the above-mentioned problems, the present inventionhas been made to provide a forging method, a forged-product moldingapparatus, and a tripod type constant velocity universal joint whichcontribute to reduction of shear drop and avoidance of an increase inprocessing load generated through contact of the leading end surface(shaft leading end) of the shaft portion and the dies even when anexcessively-heavy raw material is mixed so that a life of the die can beprolonged.

Solution to Problems

A forging method according to the present invention, in which afull-enclosed forging die including openable/closable dies and punchesfor pressing a material between the openable/closable dies is used sothat a product including a boss portion and shaft portions protrudedradially from the boss portion is molded, includes: putting, aftermolding a product-to-be-preliminarily-molded to form a preliminarilymolded product as the material, the preliminarily molded product intothe full-enclosed forging die so that the product is molded with thefull-enclosed forging die; forming, on a preliminary leading-end surfaceto be formed as a leading end surface of each of the shaft portions ofthe product formed of the preliminarily molded product, a shear-dropsuppressing portion for suppressing thickness reduction on aboss-portion side of an outer peripheral portion of the leading endsurface of each of the shaft portions of the product; and forming, at atime of putting the preliminarily molded product into the full-enclosedforging die so as to mold the product, a relief portion between theleading end surface of each of the shaft portions to be formed and acavity of the full-enclosed forging die.

According to the forging method of the present invention, the shear-dropsuppressing portion is formed on the outer peripheral portion of theleading end surface of each of the shaft portions of the product formedof the preliminarily molded product put in the full-enclosed forgingdie. Thus, at the time of molding of the shaft portions with thefull-enclosed forging die, shear drop can be reduced even under a statein which a central portion of each of the shaft portions is liable to befluidized in comparison with a peripheral portion thereof.

Further, the relief portion is formed between the leading end surface ofeach of the shaft portions to be formed and the cavity of thefull-enclosed forging die. Thus, even when an excessively-heavy rawmaterial is mixed, a gap can be sufficiently secured between the shaftleading end and the dies.

The shear-drop suppressing portion may be formed with the preliminaryleading-end surface of the preliminarily molded product being formed asa flat-surface portion, or with a curvature center of the preliminaryleading-end surface of the preliminarily molded product being arrangedon a radially outer side relative to the preliminary leading-end surfaceso that a circumferential end portion of the preliminary leading-endsurface of the preliminarily molded product has a swelling shape.

The preliminarily molded product is, for example, a hexagonal columnarbody having six surfaces as an outer peripheral surface thereof, inwhich, of the six surfaces, surfaces arranged at pitches of 120° eachconstitute the preliminary leading-end surface, and other surfaces ofthe six surfaces constitute preliminary radially-outer surfaces eachforming a boss-portion radially-outer surface between the shaftportions. There is a case where the product is a tripod member for atripod type constant velocity universal joint.

The forged-product molding apparatus according to the present inventionincludes: a full-enclosed forging die for product molding, whichincludes openable/closable dies and punches for pressing a materialbetween the openable/closable dies, which is capable of molding aproduct including a boss portion and shaft portions protruded radiallyfrom the boss portion, and in which a relief portion is formed between aproduct-molding cavity of the full-enclosed forging die and each of theshaft portions at a time of molding; and a preliminarily-molded-productdie for molding a product-to-be-preliminarily-molded as the materialincluding a shear-drop suppressing portion for suppressing thicknessreduction toward a boss-portion side of an outer peripheral portion of aleading end surface of each of the shaft portions of the product, theshear-drop suppressing portion being formed on a preliminary leading-endsurface to be formed as the leading end surface of each of the shaftportions of the product.

According to the forged-product molding apparatus of the presentinvention, the shear-drop suppressing portion is formed on the outerperipheral portion of the leading end surface of each of the shaftportions of the product formed of the preliminarily molded product putin the full-enclosed forging die. Thus, at the time of molding of theshaft portions with the full-enclosed forging die, shear drop can bereduced even under the state in which the central portion of each of theshaft portions is liable to be fluidized in comparison with theperipheral portion thereof. Further, the relief portion is formedbetween the leading end surface of each of the shaft portions to beformed and the cavity of the full-enclosed forging die. Thus, even whenan excessively-heavy raw material is mixed, the gap can be sufficientlysecured between the shaft leading end and the dies. That is, accordingto the forged-product molding apparatus of the present invention, theabove-mentioned forging method of the present invention can be stablyperformed.

A regulating portion with which the leading-end outer peripheral portionof each of the shaft portions comes into contact at the time of moldingmay be provided in the product-molding cavity of the full-enclosedforging die. When the regulating portion is provided in this manner, thematerial comes into contact with the contact portion at the time ofpressing with the punches. As a result, a shape of a part or an entireof the outer periphery of each of the shaft leading ends is secured. Thesecured part can be used as a reference plane for phase determination atthe time of a high-accuracy machining process on the shaft-portion outerperipheral surface.

In a preliminarily-molded-product-formation cavity of thepreliminarily-molded-product die, a flat-surface portion for molding ofthe shear-drop suppressing portion may be provided or a circular-arcsurface portion for molding of the shear-drop suppressing portion may beprovided, the circular-arc surface portion being convex to apreliminarily-molded-product-formation cavity side.

A tripod type constant velocity universal joint according to the presentinvention includes a tripod member molded with the forged-productmolding apparatus described above.

Advantageous Effects of Invention

In the present invention, “shear drop” at each of the shaft portions canbe reduced, and the constant velocity universal joints and the universaljoints using the forged products can be compactified and weight-reduced.In addition, even when an excessively-heavy raw material is mixed, thegap can be sufficiently secured between the shaft leading end(shaft-portion leading-end surface) and the dies, and an increase inprocessing load generated through contact of the shaft leading end andthe dies can be avoided. As a result, the life of the full-enclosedforging die can be prolonged.

The shear-drop suppressing portion can be easily formed, for example,with the preliminary leading-end surface of the preliminarily moldedproduct being formed as a flat-surface portion, or with the outerperipheral side of the preliminary leading-end surface of thepreliminarily molded product being formed in a swelling shape. Further,for example, when being constituted by a hexagonal columnar body havingsix surfaces as an outer peripheral surface, the preliminary moldedproduct can be stably molded as a tripod member of a tripod typeconstant velocity universal joint as a product.

When the regulating portion is provided with which the leading-end outerperipheral portion of each of the shaft portions comes into contact atthe time of molding, each of the parts secured by the dies can be usedas the reference plane for phase determination at the time of ahigh-accuracy machining process. Thus, it is unnecessary to remove theshaft leading end by a machining process prior to heat treatment so thatthe reference plane (reference portion) is formed. As a result, materialcost and machining-process cost can be reduced.

The tripod type constant velocity universal joint of the presentinvention can be provided with the tripod member molded with theforged-product molding apparatus, and hence can be compactified andweight-reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A A sectional view of a full-enclosed forging die of afull-enclosed forging apparatus that uses a preliminarily molded productmolded by a preliminarily-molding die, illustrating an embodiment of thepresent invention.

FIG. 1B A sectional view of a full-enclosed forging die of afull-enclosed forging apparatus that uses a preliminarily molded productmolded by a conventional preliminarily-molding die.

FIG. 2A A sectional view of a part of the full-enclosed forging die, thepart being different from that illustrated in FIG. 1A.

FIG. 2B A sectional view of a part of the full-enclosed forging die, thepart being different from that illustrated in FIG. 1B.

FIG. 3A A sectional plan view of the full-enclosed forging dieillustrated in FIG. 1A.

FIG. 3B A sectional plan view of the full-enclosed forging dieillustrated in FIG. 1B.

FIG. 4A A main-part enlarged sectional view of a product molded with thefull-enclosed forging die illustrated in FIG. 1A.

FIG. 4B A main-part enlarged sectional view of a product molded with thefull-enclosed forging die illustrated in FIG. 1B.

FIG. 5A A sectional view of the preliminarily-molding die of thefull-enclosed forging apparatus, illustrating the embodiment of thepresent invention.

FIG. 5B A sectional view of the conventional preliminarily-molding die.

FIG. 6 A sectional plan view of the preliminarily-molding dieillustrated in FIG. 5B.

FIG. 7A A processing diagram illustrating a forging method.

FIG. 7B A processing diagram illustrating the forging method.

FIG. 7C A processing diagram illustrating the forging method.

FIG. 7D A processing diagram illustrating the forging method.

FIG. 7E A processing diagram illustrating a forgoing method according tothe present invention.

FIG. 7F A processing diagram illustrating the forging method accordingto the present invention.

FIG. 8 A sectional plan view illustrating another embodiment of thepreliminarily-molding die.

FIG. 9 A sectional view of a conventional full-enclosed forging die.

FIG. 10A A processing diagram illustrating a conventional forgingmethod.

FIG. 10B A processing diagram illustrating the conventional forgingmethod.

DESCRIPTION OF EMBODIMENTS

In the following, description is made if an embodiment of the presentinvention with reference to FIGS. 1 to 7.

A forged-product molding apparatus according to the present inventionincludes a full-enclosed forging die 30 illustrated in FIGS. 1 to 3 anda preliminarily-molded-product die 31A illustrated in FIGS. 5A and 8.The full-enclosed forging die 30 includes openable and closable dies 11and 12 and punches 14 and 15 driven along an opening/closing directionof the dies 11 and 12 so as to press a material (preliminarily moldedproduct) in the dies 11 and 12. The full-enclosed forging die 30 in thiscase molds a product (for example, a tripod member for constant velocityuniversal joints) 16 radially provided with shaft portions 17. Notethat, the tripod member as the product 16A, 16B includes a boss portion18 and the three shaft portions 17 extending radially outward from theboss portion 18. The product 16A is a product using a preliminarilymolded product 20A and molded by the forged-product molding apparatus ofthe present invention, that is, the preliminarily-molded-product die 31Ashown in FIG. 8, and the product 16B is a product using a preliminarilymolded product 20C and molded by a preliminarily-molded-product die 31Bshown in FIG. 5B and FIG. 6.

Guide holes 21 a and 21 b are provided at respective axial portions ofthe dies 11 and 12, and the punches 14 and 15 are fit-insertedrespectively into the guide holes 21 a and 21 b. Further, at an openingportion of each of the guide holes 21 a and 21 b on respectivemating-surface-11 a-and-12 a sides of the dies 11 and 12, there arerespectively arranged three recessed portions 22 and 23 alongcircumferential directions at pitches of 120° while extending in radialdirections. Still further, a swelling portion 27 is provided at acentral portion of a lower surface 14 a of the upper punch 14, and aswelling portion 28 is provided at a central portion of an upper surface15 a of the lower punch 15.

Under a state in which the dies 11 and 12 are superimposed on each otheras illustrated in FIG. 1, shaft-portion spaces 24A for forming the shaftportions 17 of each of the products 16A, 16B are formed of the recessedportions 22 and 23 facing each other. Further, a boss-portion space 24Bfor molding the boss portion 18 is formed of the punches 14 and 15 andthe dies 11 and 12. That is, a cavity 24 for product molding isconstituted by the shaft-portion spaces 24A and the boss-portion space24B. In this case, on a radially outer side in each of the spaces 24A,contact portions 25 swell into the space 24A so as to come into contactwith a leading end side of an outer peripheral surface of each of theshaft portions 17 to be formed. In addition, a gap (relief portion) 26is provided between the contact portions 25 and a leading end surface 17a of each of the shaft portions 17 to be formed.

As illustrated in FIGS. 5A and 5B, the preliminarily-molded-product dies31A, 31B each includes a preliminary-molding die 32, apreliminary-molding punch 33 and an ejector 34, which are fit-insertedinto a hole portion 32 a of the preliminary-molding die 32.

The hole portion 32 a of the preliminary-molding die 32 is a hexagonalhole including a peripheral wall 50 having, as illustrated in FIG. 6,circular-arc surface portions 51 (51 a, 51 b, and 51 c) and flat-surfaceportions 52 (52 a, 52 b, and 52 c), which are alternately arranged. Inthis case, a material 20B put in the hole portion 32 a of thepreliminary-molding die 32 is a columnar body illustrated in FIG. 7A.

As illustrated in FIG. 7A, a radius R2 of a radially outer surface ofthe material 20B is set to such an extent that the radially outersurface comes into contact with the flat-surface portions 52 a, 52 b,and 52 c. That is, the flat-surface portions 52 a, 52 b, and 52 c serveas tangents with respect to the radially outer surface. Thus, under thestate in which the material 20B is put in the hole portion 32 a of thepreliminary-molding die 32, gaps are formed at the circular-arc surfaceportions 51 a, 51 b, and 51 c, and at corner portions respectivelybetween the flat-surface portions 52 a, 52 b, and 52 c, and thecircular-arc surface portions 51 a, 51 b, and 51 c.

Further, as illustrated in FIG. 5, a swelling portion 35 is formed at acentral portion of a lower surface 33 a of the preliminary-molding punch33, and a swelling portion 36 is formed at a central portion of an uppersurface 34 a of the ejector 34. The swelling portion 35 of thepreliminary-molding punch 33 has the same dimension and shape as thoseof the swelling portion 27 of the above-mentioned upper punch 14, andthe swelling portion 36 at the central portion of the ejector 34 has thesame dimension and shape as those of the swelling portion 28 of theabove-mentioned lower punch 15. Note that, the swelling portion 35 ofthe preliminary-molding punch 33 and the swelling portion 36 at thecentral portion of the ejector 34 may be same as or different from eachother in shape, and the swelling portion 27 of the punch 14 and theswelling portion 28 of the punch 15 may be same as or different fromeach other in shape. Note that, a reinforcing member (reinforcing ring)(not shown) is externally fitted to the preliminary-molding die 32 bypress fitting, shrink fitting, or the like.

Next, description is made of a method of forming a material(preliminarily molded product) 20C with use of the die apparatus 31B.First, as illustrated in FIG. 7A, a disk-like billet 20B is put into thedie apparatus 31B in an open state. In this case, the open staterepresents a state in which the preliminary-molding punch 33 is raised,that is, a state in which the billet 20B can be put into the holeportion 32 a of the preliminary-molding die 32. Further, although notshown, the material 20A 20C may be finally obtained through an ironingprocess on an outer peripheral surface of the billet 20B. The billet 20Bis inserted into the hole portion 32 a with a gap ranging from φ0.005 toφ0.3. Still further, when the outer peripheral surface of the billet 20Bis formed by the ironing process, it suffices to provide, on abillet-putting-in side of the preliminary-molding die 32, a guideportion which allows the billet 20B to be inserted with theabove-mentioned gap.

In this state, the preliminary-molding punch 33 is lowered so that thebillet 20B is pressed with the preliminary-molding punch 33 and theejector 34. With this, the billet 20B is plastically deformed so as tofill a cavity 38 constituted by the hole portion 32 a of thepreliminary-molding die 32, the preliminary-molding punch 33, and theejector 34. As a result, as illustrated in FIG. 7B, the material 20C isformed. Specifically, the material 20C can be formed, which has ahexagonal surface constituted by a radially outer surface 60 includingcircular-arc surface portions 61 (61 a, 61 b, and 61 c) and flat-surfaceportions 62 (62 a, 62 b, and 62 c) which are alternately arranged.

Next, description is made of a forging method with use of thefull-enclosed forging die illustrated in FIGS. 1 to 3. First, the upperdie 11 and the lower die 12 are brought into such a die-open state as tobe separated relatively to each other. In this case, the upper punch 14is raised and the lower punch 15 is lowered. In this state, each of thepreliminarily molded products 20A, 20C is put into the guide hole 21 bof the lower die 12 in advance. Note that, each of the preliminarilymolded products 20A, 20C can be fit-inserted into the guide holes 21 aand 21 b, and corresponds to a volume of a product to be formed. Thepreliminarily molded product 20A is molded by the preliminarily-moldingdie 31A of FIG. 8 to be described later.

After that, clamping is performed so as to bring the upper die 11 andthe lower die 12 close relatively to each other. Next, the upper punch14 is lowered, and simultaneously the lower punch 15 is raised. Withthis, the above-mentioned spaces 24A are formed in which thepreliminarily molded product 20A is pressed from above and below so thatthe shaft portions 17 are formed. A preliminarily molded product 20A isfluidized in the spaces 24A so that each of the products 16A, 16B(tripod member) including the three shaft portions 17 radially aroundthe boss portion 18 as illustrated in FIGS. 4A and 4B is formed.

That is, in the forged-product molding apparatus of the presentinvention, first, as illustrated in FIG. 7A, the material 20B is putinto the preliminary-molding dies 31A, 31B so that the preliminarilymolded products 20A, 20C are molded as illustrated in FIGS. 8 and 7B.After that, the preliminarily molded products 20A, 20C are put into thefull-enclosed forging dies 30, 30 as illustrated in FIGS. 7E and 7C sothat the product is molded as illustrated in FIGS. 7D and 7F.

In this case, in the preliminarily molded product 20C, preliminaryleading-end surfaces to be formed as the leading end surfaces 17 a ofthe shaft portions 17 molded with the die are constituted respectivelyby the above-mentioned flat-surface portions 62 (62 a, 62 b, and 62 c).In addition, the flat-surface portions 62 (62 a, 62 b, and 62 c) extendin tangential directions of the material 20B, and hence circumferentialend portions of the flat-surface portions 62 (62 a, 62 b, and 62 c) donot recede to a radially inner side. Thus, the flat-surface portions 62constitute shear-drop suppressing portions 65 for suppressing thicknessreduction toward a boss-portion-18 side of the outer peripheral portionsof the shaft portions 17. That is, when the shaft portions 17 are moldedwith the full-enclosed forging die 30, shear drop can be reduced evenunder a state in which a central portion of each of the shaft portions17 is liable to be fluidized in comparison with a peripheral portionthereof. Thus, constant velocity universal joints and universal jointsusing the forged product can be compactified and weight-reduced. Inaddition, the relief portion is formed between the leading end surface17 a of each of the shaft portions 17 to be formed and the cavity 24 ofthe full-enclosed forging die 30. Thus, even when an excessively-heavyraw material is mixed, a gap can be sufficiently secured between theleading end surface (shaft leading end) 17 a of each of the shaftportions 17 and the dies 11 and 12, and hence a processing-load increasecaused by contact of the shaft leading end 17 a and the dies 11 and 12can be avoided. As a result, a life of the full-enclosed forging die 30can be prolonged.

When a regulating portion 25 is provided, with which the leading-endouter peripheral portion of the shaft portion 17 comes into contact atthe time of molding, parts secured by the dies 11 and 12 each can beused as a reference plane for phase determination at the time of ahigh-accuracy machining process. Thus, it is unnecessary to remove theshaft leading end 17 a by a machining process prior to heat treatment sothat the reference plane (reference portion) is formed. As a result,material cost and machining-process cost can be reduced.

The preliminarily-molding die 31A may be provided with circular-arcsurface portions 55 for molding shear-drop suppressing portions convexto a cavity-38 side as illustrated in FIG. 8. In this case, the cavity38 illustrated in FIG. 8 is obtained through replacement of theflat-surface portions 52 a, 52 b, and 52 c in the cavity 38 of thepreliminarily-molding die 31B illustrated in FIG. 6 with thecircular-arc surface portions 55 a, 55 b, and 55 c. Each of thecircular-arc surface portions 55 a, 55 b, and 55 c has a curvaturecenter arranged on an outside of the cavity 38.

Thus, the preliminarily molded product 20A can be molded even with thepreliminarily-molding die 31 having the cavity 38 described above. Thepreliminarily molded product 20A has a hexagonal surface constituted bythe radially outer surface 60 including circular-arc surface portions(convex circular-arc surface portions) 63 (63 a, 63 b, and 63 c) andcircular-arc surface portions (concave circular-arc surface portions) 64(64 a, 64 b, and 64 c) which are alternately arranged. Thus,circumferential end portions of the circular-arc surface portions 64each have a swelling shape.

Thus, when the shaft portions 17 are molded with the full-enclosedforging die 30 under a state in which each of the swelling portions ofthe circular-arc surface portions constitutes the shear-drop suppressingportion 65, shear drop can be reduced even under the state in which thecentral portion of each of the shaft portions 17 is liable to befluidized in comparison with the peripheral portion thereof. A curvatureradius R1 of each of the preliminary leading-end surfaces 55 a, 55 b,and 55 c (64 a, 64 b, and 64 c) may be variously set as long as each ofthe circumferential end portions has a swelling shape. Note that, incomparison with such a case as to be constituted by a flat-surfaceportion, the shear-drop suppressing portion 65 constituted by acircular-arc surface has such an advantage as to be capable of savingmanufacturing cost of the punch 33 and the ejector 34.

Hereinabove, although description is made of the embodiments of thepresent invention, the present invention is not limited to theembodiments described above, and various modifications may be madethereto. For example, although the full-enclosed forging die 30 in theembodiments includes the plurality of contact portions 25 arranged atpredetermined pitches along the circumferential direction, a singlecontact portion 25 may be formed over the entire periphery of each ofthe spaces 24A. Further, sectional shapes and sizes of the contactportions 25 may be arbitrarily changed as long as outer peripheralshapes of the shaft leading ends are secured by the dies 11 and 12 andeach of the secured parts can be used as a reference plane for phasedetermination at the time of a high-accuracy machining process.

Further, in the full-enclosed forging die illustrated in FIG. 1, theswelling portions 27 and 28 of the upper and lower punches 14 and 15 maybe different from or same as each other in shape. Still further, in thedie apparatus 31 illustrated in FIGS. 5 and 6, although being same aseach other, the swelling portions 35 and 36 of the preliminary-moldingpunch 33 and the ejector 34 may be different from each other in shape.

Example 1

Next, comparison of a “shear drop” state was made between a case wherethe preliminary molding illustrated in FIGS. 7A to 7D was performed anda case where the preliminary molding was not performed. Table 1represented below shows the results of the comparison. In Table 1,“billet curvature radius R2” represents a curvature of the material 20Bprior to the preliminary molding, “pre-molding curvature radius R1”represents a curvature radius of the preliminary leading-end surfaces ofthe preliminarily molded material 20C, “shaft-end curvature radius R3”represents, as illustrated in FIG. 4B, a curvature radius of the leadingend surface 17 a of the formed shaft portion 17, and “shear drop”represents a difference between an outermost top of the leading endsurface 17 a of the formed shaft portion 17 and an outer edge.

In Table 1, “product of the present invention” represents a productusing the preliminarily molded product 20C having the flat-surfaceportions 62 illustrated in FIG. 7A, “conventional product” represents aproduct free from being preliminarily molded, and “comparison product”represents a product having circular-arc surfaces convex on the radiallyouter side in place of the flat surface portions 62, the circular-arcsurfaces each having a curvature radius of 47.8 mm and a shaft-endcurvature radius R3 of 30.5 mm.

TABLE 1 Product of the Comparison Conventional present invention productproduct Pre-molding No pre-molding Billet curvature 16.0 radius R2Pre-molding Linear 47.8 curvature radius R1 Shaft-end 40.0 30.5 22.1curvature radius R3 Shear drop 0.7 1.4 2.1

Table 1 shows that shear drop was successfully suppressed. Specifically,shear drop was 2.1 mm in a case where the material is inserted into amid-molding die and processed without pre-molding, and shear drop was0.7 mm in a case where the material is processed in the mid-molding dieafter being pre-molded. Note that, shear drop was 1.4 mm in thecomparison example.

INDUSTRIAL APPLICABILITY

The present invention is used for a full-enclosing method of forming aproduct including a boss portion radially provided with shaft portions,such as a trunnion (tripod member) for constant velocity universaljoints and a cross spider for universal joints.

REFERENCE SIGNS LIST

11, 12 die

16 product

17 shaft portion

17 a leading end surface

18 boss portion

20A preliminarily molded product

26 relief portion

30 full-enclosed forging die

31 preliminarily-molded product die

51 (51 a, 51 b, 51 c) circular-arc surface portion

52 (52 a, 52 b, 52 c) flat-surface portion

55 (55 a, 55 b, 55 c) circular-arc surface portion

61 (61 a, 61 b, 61 c) circular-arc surface portion

62 (62 a, 62 b, 62 c) flat-surface portion

63 (61 a, 61 b, 61 c) circular-arc surface portion

64 (62 a, 62 b, 62 c) flat-surface portion

The invention claimed is:
 1. A forged-product molding apparatus comprising: a full-enclosed forging die for product molding, which comprises openable/closable dies and punches for pressing a material between the openable/closable dies, which is capable of molding a product comprising a boss portion and shaft portions protruded radially from the boss portion, and in which a relief portion is formed between a cavity and each of the shaft portions at a time of molding; and a preliminarily-molded-product die having a preliminarily-molded-product-formation cavity, the preliminarily-molded-product die for molding a product-to-be-preliminarily-molded as the material, which comprises a shear-drop suppressing portion for suppressing thickness reduction toward a boss-portion side of an outer peripheral portion of a leading end surface of each of the shaft portions of the product, the shear-drop suppressing portion being formed on a preliminary leading-end surface to be formed as the leading end surface of each of the shaft portions of the product, wherein a product-molding cavity of the full-enclosed forging die has provided therein a regulating portion with which a part or an entirety of a leading-end outer peripheral portion of each of the shaft portions comes into contact at the time of molding, and the preliminarily-molded-product-formation cavity of the preliminarily-molded-product die has provided therein a circular-arc surface portion for molding of the shear-drop suppressing portion, the circular-arc surface portion being convex toward the preliminarily-molded-product-formation cavity. 